The dyeing of textile materials is not an efficient process, as much of the dye used to dye textiles remains in the dyeing medium. Because the exact composition of the partially-depleted dyeing medium varies, reuse of the depleted dyeing medium may produce inconsistent results. As a consequence, this residual dyeing medium is typically discarded, resulting in the release of large amounts of dye-contaminated wastewater into the environment. Colored industrial effluent can cause considerable damage to the ecosystem, e.g., due to their effects on photosynthetic activity in aquatic life due to reduced light penetration.
Conventional treatment techniques applied to textile wastewater for dye removal include physical methods (coagulation/flocculation), membrane separation (ultrafiltration, reverse osmosis), and elimination by activated carbon adsorption. Such methods are costly and also result in phase transfer of pollutants. Current microbiological methods are not an efficient solution to treatment of the wastewater due to the complex structures of some dyes that render them resistant to biodegradation.
Chemical decomposition of dye is generally carried out in two distinct ways. Dye may be partially removed using dilute hydrochloric acid or formic acid solutions in a process known as “brightening.” Alternatively, dye may be decomposed using reducing agents, oxidizing agents and discharging assistants, in a process known as “stripping.”
Oxidation using hydrogen peroxide (H2O2) is commonly used because cleaner byproducts are generated in comparison to agents such as chlorine or conventional bleach (i.e., hypochlorite). H2O2 is known to be ineffective with anthraquinone or azo dyes, which are widely-used and have a significant environmental impact. In addition, the decolorization reaction with H2O2 is slow. Faster oxidative decolorization methods utilize advanced oxidative processes (AOPs) process, which typically use ozone (O3), ozone activated with ultraviolet light (UV)/O3, hydrogen peroxide activated with UV (UV/H2O2), or “activators” that generate stronger oxidizing agents such as peracids. Such processes can be prohibitively expensive on a large scale, e.g., due to the cost of chemical activators or ultraviolet energy.
Stripping is typically performed with alkaline sodium dithionite, sodium chlorate, sodium hypochorite or thioureadioxide. These chemicals are environmentally unfriendly and from a health and safety aspect need careful handling.
The need exists for more efficient and environmentally friendly methods for decolorization of dyes.